Pershing Square Holdings, the publicly traded vehicle led by Ackman, faltered in August because of "significant volatility in the investment markets ... largely driven by the decline of the Chinese stock markets, and the fear that slowing growth in China will have repercussions for businesses around the world," he wrote in an investor update.

The fund had been up more than 10.1% through the end of July. When the market tanked, his fund wend down with it, dropping 13.1% from August 1 to 25. That put the fund in the red for the year, down 4.3% for 2015. Toward the end of the month, the fund pared back some of those losses and finished the month down 9.2%, leaving the fund down 0.1% for the year.

Ackman holds large positions in a handful of publicly traded companies such as Canadian Pacific Railway, Mondelez, and Valeant. He's also famously short Herbalife, a multilevel marketing company he believes is a "pyramid scheme."